Voltage-controlled oscillator comprising a circuit for compensating frequency pulling

ABSTRACT

The present invention relates to a method for stabilising the operation of a voltage controlled oscillator (VCO) driven by a phase locked loop (PLL), the voltage controlled oscillator delivering an RF signal and receiving through at least one spurious path a harmonic component of a frequency equal or proximate to that of the RF signal, capable of disturbing its operation by injection pulling. According to the present invention, the method comprises a step of injecting into the voltage controlled oscillator an injection pulling compensation signal, the phase and the amplitude of which are adjusted so as to neutralise the effects of the spurious harmonic component. Application particularly to phase modulation IQ circuits in radiotelephony.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to voltage controlled oscillators or VCOs.

The present invention relates more particularly to an RF circuitcomprising a voltage controlled oscillator delivering an RF signal, aphase locked loop to control the voltage controlled oscillator, amodulation circuit receiving the RF signal and delivering a modulatedsignal comprising at least one RF harmonic component capable ofdisturbing the voltage controlled oscillator by injection pulling.

2. Description of the Related Art

In radio frequency circuits using VCOs, the performances of the VCOs aredeteriorated by leaks of harmonic signals, due to “injection pulling”also known as “injection locking”.

The present invention aims to remove, or at least to reduce, theinjection pulling in the VCOs.

For a better understanding, FIG. 1 shows a classical application of aVCO in the area of radiotelephony. In this Figure a circuit RFCTcomprising a VCO, a circuit forming a phase locked loop or PLL circuitand a modulation circuit TXCT can be distinguished.

The VCO delivers to the modulation circuit TXCT a voltage V1 thefrequency F1 of which is controlled by the PLL circuit. For thatpurpose, the PLL circuit comprises a divide-by-N frequency divider DIVNthat receives the voltage V1 at input and that delivers a signal offrequency F1/N to one input of a phase comparator PCOMP. The phasecomparator receives a reference frequency F_(REF) at another input. Thisfrequency F_(REF) is, for example, delivered by a divide-by-M dividerDIVM the input of which is linked to a quartz oscillator. The output ofthe comparator delivers a control signal Vcont that is applied to onecontrol input of the VCO through a loop filter LOOPF having a determinedbandwidth. The signal V1 is therefore frequency and phase controlled andits frequency F1 is equal to N/M*F_(REF).

Here the circuit TXCT is a data transmission circuit by phase modulationIQ (quadrature modulation PM) provided for a mobile telephone forexample.

The circuit TXCT receives an analog signal Sx and the signal V1 from theVCO at input, and delivers a signal RFSX intended to be applied to an RFantenna, that is phase modulated by means of two quadrature signals Iand Q.

The circuit TXCT comprises a divide-by-K divider DIVK the input of whichreceives the signal V1 and the output of which delivers a modulation PMcarrier F_(RF), F_(RF) being equal to F1/K, K generally being equal to 2or to 4. The signal Sx is digitised by a converter ADC, then it isapplied to an encoder modem CODEM then is applied again to a processorIQGEN. The processor IQGEN delivers, in a baseband of frequency F_(BB),phase I and quadrature Q signals. The signal I is applied to one inputof a mixer IMIX through an amplifier IAMP, and the signal Q applied toone input of a mixer QMIX through an amplifier QAMP. The mixer IMIXreceives the carrier F_(RF) at another input and the mixer QMIX receivesthe carrier F_(RF) phase-shifted by 90° at another input, delivered by aphase shifter DPH. The outputs of the mixers IMIX, QMIX are applied toan adder IQAD that delivers the modulated signal RFSx. The signal RFSxis applied to an output amplifier RFAMP the output of which forms theoutput of the transmission circuit TXCT.

The signal Sx generally contains data to be transmitted, such as a codedvoice for example, and has a spectrum of frequencies representative ofthe modulation schema provided for by the standard implemented (such asGMSK in GSM for example). Considering, as an example, that the signal Sxis a single tone, the circuit IQGEN then delivers two pure quadraturesine curves I=cos (F_(BB)) and Q=sin (F_(BB)). The result of the phasemodulation IQ is, in this case, a single tone of frequency F_(RF)+F_(BB)the image component F_(RF)−F_(BB) of which is removed by the quadraturemodulation, and the carrier F_(RF) of which is also removed.

Due to imperfections in the modulation circuit, or “non-linearity”, theoutput signal comprises in addition to the wanted component H1 offrequency F_(RF)+F_(BB), harmonics H2, H3, H4, . . . . At least one ofthese components is proximate to the oscillation frequency F1 of theVCO. It is the first harmonic H1 (wanted component) when the dividerDIVK does not exist or has a division value equal to 1 (K=1), the secondharmonic H2 when the divider DIVK is a divide-by-2 divider (K=2) or thefourth harmonic H4 when the divider DIVK is a divide-by-4 divider (K=4).When K=2, the frequency of the second harmonic H2 is in fact equal to2F_(RF)+2F_(BB) (i.e., F1+2F_(BB)) and is very proximate to the centrefrequency F1 of the VCO as the frequency of the baseband F_(BB) is lowbefore the carrier F_(RF), generally in the order of a few Gigahertz.Similarly, when K=4, the fourth harmonic H4 has a frequency of4F_(RF)+4F_(BB) (i.e., F1+4F_(BB)) that is proximate to the centrefrequency of the VCO.

It is well known that the involuntary injection of this harmoniccomponent into the core of the VCO, by various spurious paths,deteriorates the performances of the VCO.

Various methods are known to overcome this disadvantage.

One known method involves producing the VCO on a substrate distinct fromthe one bearing the phase modulation IQ circuit TXCT. This substrate isarranged in a sheathed case and comprises means for connecting to thecircuit TXCT that are equipped with insulating barriers preventing thespurious harmonics sent by the circuit TXCT from “rising” to the core ofthe VCO. These barriers generally comprise filters, “balun” typeconnectors, insulators, buffer circuits . . . and must be provided inall the conduction paths linking the VCO to the circuit TXCT, includingthe power supply paths. This solution is however complex to implementand increases the cost price of the RF circuits, which is passed on atthe end of the chain to the selling price of the mobile telephones.

Other methods are based on providing a phase modulation IQ circuitarchitecture in which the VCO is quite insensitive to the spuriousharmonics.

Therefore, the heterodyne systems use several VCOs and severalcascade-arranged mixers, and a premodulation stage using an intermediatefrequency IF. In the output stage, the frequency of the modulated signalis clearly offset in relation to the natural frequency of the VCO, andthe harmonics capable of interfering with the VCO are harmonics and/ormixing products of high rank that are greatly attenuated.

However, the disadvantage of the heterodyne systems is that they requirethe use of at least two VCOs, as well as additional mixers and filters,and are therefore costly and bulky.

Another solution to counter the injection pulling includes providing acopy loop in the VCO. This copy loop allows harmonic frequencies to beobtained that are offset in relation to the centre frequency of the VCO,and are located outside its bandwidth (determined by the loop filter).However, this solution also requires using several VCOs, generally threeVCOs at least.

Various architectures of RF modulation circuits or of VCOs that arequite insensitive to injection pulling are described particularly inU.S. Pat. Nos. 6,321,1074, 5,144,260 and 6,281,758.

BRIEF SUMMARY OF THE INVENTION

The present invention aims to provide a quite different method forremoving or limiting the injection pulling in the VCOs, that is simpleand inexpensive to implement, and that can provide good results in amodulation circuit that only uses a single VCO, whether it is a phasemodulation IQ circuit or an amplitude modulation circuit or even a phaseand amplitude modulation circuit.

To achieve this object, the present invention is based on an in-depthstudy of the disturbance mechanisms occurring in a VCO, that will bedescribed below. At the end of this study, and as it will be seen ingreater detail subsequently, the conclusion could be drawn that theinjection pulling is attributable to the injection into the VCO of aspurious harmonic that passes along many spurious paths each havingtheir own transfer function, thus forming a plurality of spurioussignals. These spurious signals are added to each other and there is asingle resulting spurious signal that is the result of the vector sum ofthe spurious signals.

The conclusion could also be drawn that the resulting spurious signal isthe sole cause of all the forms of disturbance attributable to theinjection pulling, and that by removing it, or at least attenuating itas far as possible, the injection pulling can be removed, or at leastreduced sufficiently with regard to the expected specifications of an RFmodulation circuit.

Therefore, the principle of the present invention is to voluntarilyinject into a VCO a spurious signal that has the same amplitude as theresulting spurious signal injected involuntarily but which is inopposite phase with the latter, such that the vector sum of theresulting spurious signal injected involuntarily and of the spurioussignal injected voluntarily is equal to 0. This spurious signal injectedvoluntarily forms a compensation signal according to the presentinvention that neutralises the injection pulling in a VCO.

Another principle of the present invention is to generate thecompensation signal by taking off the disturbing harmonic in themodulation circuit itself, at a point rich in harmonics, then byapplying this harmonic to a phase and amplitude control circuit so as todeliver the compensation signal.

More particularly, the present invention relates to a method forstabilising the operation of a voltage controlled oscillator driven by aphase locked loop, the voltage controlled oscillator delivering an RFsignal and receiving through at least one spurious path a harmoniccomponent of a frequency equal or proximate to that of the RF signalsent, capable of disturbing the operation of the voltage controlledoscillator by injection pulling, comprising the injection, into thevoltage controlled oscillator, of an injection pulling compensationsignal, the phase and the amplitude of which are adjusted so as toneutralise the disturbing effects of the harmonic component.

According to one embodiment, the compensation signal is amplitude andphase adjusted so as to have an amplitude substantially equal to theamplitude of a spurious signal resulting from the involuntary injectioninto the voltage controlled oscillator, by at least one spurious path,of the disturbing harmonic component, and a phase opposite that of thespurious signal.

According to one embodiment, the method comprises the single endedinjection, at one point of the voltage controlled oscillator, of acompensation signal having a unique component.

According to one embodiment, the method comprises the injection of acompensation signal having two components, and the single endedinjection of these components at two different points of the voltagecontrolled oscillator.

According to one embodiment, the method comprises the injection of acompensation signal having two components in opposite phase, and theinjection of these two components at two different points of the voltagecontrolled oscillator.

According to one embodiment, the compensation signal is generated fromat least one harmonic component taken off in the modulation circuit.

According to one embodiment, the compensation signal is generated fromat least one harmonic component taken off in an amplifier of amodulation circuit from which the disturbing harmonic component is sent.

According to one embodiment, the compensation signal is generated fromone harmonic component produced by a harmonic generating circuit.

According to one embodiment, the phase of the compensation signal isadjusted by means of a phase-shift circuit.

According to one embodiment, the amplitude of the compensation signal isadjusted by means of an attenuator circuit comprising adjustableresistors or capacitors or a combination of these elements.

According to one embodiment, the amplitude and the phase of thecompensation signal are adjusted by means of a group of at least twoattenuator circuits the outputs of which are added up.

According to one embodiment, the amplitude and the phase of thecompensation signal are adjusted by means of a group of attenuatorcircuits having their outputs added up and receiving at input phasequadrature signals coming from the disturbing harmonic component.

According to one embodiment, the amplitude and the phase of thecompensation signal are adjusted by means of a group of attenuatorcircuits having their outputs added up and receiving at input phasequadrature and opposite phase signals coming from the disturbingharmonic component.

According to one embodiment, the phase quadrature and opposite phasesignals are generated by means of a phase-shift circuit comprising abalanced bridge of resistors and capacitors that is quite insensitive tothe temperature.

According to one embodiment, an attenuator circuit compriseselectrically adjustable capacitors or electrically adjustable resistorsthat are adjusted by analog signals coming from adjustment digital data.

According to one embodiment, the adjustment digital data are stored inmemory cells.

According to one embodiment, the compensation signal is injected ontoone terminal of an active component of the voltage controlledoscillator.

According to one embodiment, the compensation signal is injected ontoone terminal of a passive component of the voltage controlledoscillator.

According to one embodiment, the compensation signal is injected byinductive coupling.

The present invention also relates to an RF circuit comprising a voltagecontrolled oscillator delivering an RF signal, a phase locked loop tocontrol the voltage controlled oscillator, a modulation circuitreceiving the RF signal and delivering a modulated signal comprising atleast one harmonic component of a frequency equal or proximate to thatof the RF signal delivered by the voltage controlled oscillator, theharmonic component being capable of disturbing the operation of thevoltage controlled oscillator by injection pulling, the RF circuitcomprising an injection pulling compensation circuit comprising oneinput receiving at least the disturbing harmonic component and means formodifying the phase and the amplitude of the harmonic component todeliver an injection pulling compensation signal, and means forinjecting the compensation signal into the voltage controlledoscillator.

According to one embodiment, the compensation circuit is amplitude andphase adjusted such that the compensation signal injected into thevoltage controlled oscillator has an amplitude substantially equal tothe amplitude of a spurious signal resulting from the involuntaryinjection into the voltage controlled oscillator, by at least onespurious path, of the disturbing harmonic component, and a phaseopposite that of the spurious signal.

According to one embodiment, the compensation circuit is a single endedcircuit that delivers a compensation signal having a unique componentthat is injected at one point of the voltage controlled oscillator.

According to one embodiment, the compensation circuit is a single endedcircuit that delivers a compensation signal having two components thatare injected at two different points of the voltage controlledoscillator.

According to one embodiment, the compensation circuit is a balancedcircuit that delivers a compensation signal having two components inopposite phase that are injected at two different points of the voltagecontrolled oscillator.

According to one embodiment, the compensation circuit receives at inputa harmonic component taken off in the modulation circuit.

According to one embodiment, the compensation circuit receives at inputa harmonic component taken off in an output amplifier of the modulationcircuit.

According to one embodiment, the compensation circuit receives at inputa harmonic component delivered by a harmonic generating circuit distinctfrom the modulation circuit.

According to one embodiment, the compensation circuit comprises aphase-shift circuit to modify the phase of the harmonic componentreceived at input.

According to one embodiment, the compensation circuit comprises aphase-shift circuit receiving the disturbing harmonic component anddelivering two phase quadrature signals.

According to one embodiment, the compensation circuit comprises aphase-shift circuit receiving the disturbing harmonic component anddelivering phase quadrature and opposite phase signals.

According to one embodiment, the phase-shift circuit comprises abalanced bridge of resistors and capacitors that is quite insensitive tothe temperature.

According to one embodiment, the compensation circuit comprises at leastone attenuator circuit to modify the amplitude of the harmonic componentreceived at input.

According to one embodiment, the attenuator circuit comprises adjustableresistors or capacitors or a combination of these elements.

According to one embodiment, the RF circuit comprises a group of atleast two attenuator circuits the outputs of which are added up tocontrol the phase and the amplitude of the compensation signal.

According to one embodiment, the RF circuit comprises a group ofattenuator circuits having their outputs added up and receiving at inputphase quadrature signals coming from the disturbing harmonic component.

According to one embodiment, the RF circuit comprises a group ofattenuator circuits having their outputs added up and receiving at inputphase quadrature and opposite phase signals coming from the disturbingharmonic component.

According to one embodiment, an attenuator circuit compriseselectrically adjustable capacitors or electrically adjustable resistors,which are adjusted by analog signals delivered by a digital to analogconverter.

According to one embodiment, digital data for adjusting the capacitorsof the attenuator circuit are stored in memory cells and are applied tothe digital to analog converter.

According to one embodiment, the compensation signal is injected, ontoone terminal of an active component of the voltage controlledoscillator.

According to one embodiment, the compensation signal is injected ontoone terminal of a passive component of the voltage controlledoscillator.

According to one embodiment, the means for injecting the compensationsignal comprise an injection inductor coupled to an inductor of thevoltage controlled oscillator.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be explained in greater detail in the followingdescription of the method according to the present invention and ofvarious examples of embodiments of compensation circuits according tothe present invention, given in relation with, but not limited to, thefollowing figures:

FIG. 1 represents a classical phase modulation IQ circuit comprising avoltage controlled oscillator,

FIG. 2A is the diagram of a theoretical model of voltage controlledoscillator used to analyse a frequency jumping phenomenon,

FIG. 2B is the diagram of a theoretical model of voltage controlledoscillator used to analyse a noise and spurious signal phenomenon,

FIG. 3 represents frequency jumps occurring in a voltage controlledoscillator in the presence of a switched spurious signal,

FIG. 4 represents the extent of the frequency jumps according to thephase of the spurious signal arriving in the core of the VCO,

FIG. 5A represents the spectrum of frequencies of a signal delivered bythe phase modulation IQ circuit in FIG. 1 when a quadrature sine-wavesignal is applied to it at input,

FIG. 5B represents the spectrum of frequencies of a signal present inthe voltage controlled oscillator in FIG. 1,

FIGS. 6A and 6B are vectorial representations schematically showing theappearance of an image frequency in the spectrum of frequenciesrepresented in FIG. 5B,

FIG. 7 represents a rejection curve of a spurious signal present in thevoltage controlled oscillator in FIG. 1,

FIG. 8 schematically represents a voltage controlled oscillatorcomprising a compensation circuit according to the present invention,

FIG. 9 is the partial wiring diagram of a classical voltage controlledoscillator, on which points of injection of a compensation signalaccording to the present invention are marked,

FIG. 10 represents in block form a first embodiment of a compensationcircuit according to the present invention,

FIG. 11 is the wiring diagram of a phase control element represented inblock form in FIG. 10,

FIG. 12 is the wiring diagram of an amplitude control elementrepresented in block form in FIG. 10,

FIG. 13 represents in block form a second embodiment of a compensationcircuit according to the present invention,

FIG. 14 is the wiring diagram of a quadrature signal generatorrepresented in block form in FIG. 13,

FIG. 15 is the wiring diagram of an amplitude control elementrepresented in block form in FIG. 13,

FIG. 16 is a phase diagram showing the operation of the compensationcircuit in FIG. 13,

FIG. 17 represents in block form a third embodiment of a compensationcircuit according to the present invention,

FIG. 18 is the wiring diagram of a quadrature signal generatorrepresented in block form in FIG. 17, and

FIG. 19 is the wiring diagram of an amplitude control elementrepresented in block form in FIG. 17.

DETAILED DESCRIPTION OF THE INVENTION

Experimental and Theoretical Bases of the Present Invention

As explained above, the principle of the present invention is to injecta compensation signal into a VCO that neutralises in phase and inamplitude a resulting spurious signal equal to the vector sum of theincident spurious signals coming from a harmonic component delivered bya modulation circuit.

Before describing examples of embodiments of compensation circuitsenabling this compensation signal to be generated, various experimentalobservations, theoretical studies and hypotheses having led to thepresent invention will be succinctly described.

The deterioration caused by the injection pulling results in twodistinct phenomena. Firstly, there is instantaneous frequency jumps inthe VCO, and secondly a spurious modulation leading to a substantialphase error and spurious lines in the spectrum of frequencies of theVCO. It must be shown that these two phenomena have a single causetaking the form of a single spurious signal having a determinedamplitude and phase, and that they are mutually correlated, in terms ofphase and amplitude of the spurious signal that generates them.

Experimental observations of these two phenomena have been combined withtheoretical studies and computer simulations.

In the following description, reference will be made to the example of amodulation circuit TXCT described in relation with FIG. 1. Thecoefficient K of the divider DIVK is taken to be equal to 2. In thiscase, it is the second harmonic H2 of the output signal of the amplifierRFAMP that is the component the most proximate to the centre frequencyof the VCO.

Modelling a VCO

In accordance with an accepted theory and as shown in FIGS. 2A, 2B, aVCO can be modelled in the form of two elements A and B in closed loop,A being the active part of the VCO, modelled by a gain transconductanceamplifier Gi, B being the reactive part of the VCO, modelled by aresistor, a capacitor and an inductor in parallel, i.e., an impedance F(ω) of value:F (ω)=(1/R+1/(j ω L)+j ω C)⁻¹   (1)

When the VCO is balanced (switch SWP open in FIG. 2A), the equation ofthe closed loop is written:V1=V1 Gi F (ω)   (2)i.e.:Gi=1/F (ω)   (3)

To obtain stable oscillation conditions, the centre frequency ω1 of theVCO must be equal to:ω1=1/√{square root over ( )}LC   (4)

The result is that: Gi=1/R

First Disturbing Phenomenon: Frequency Jumps in the VCO

A first phenomenon that deteriorates the performances of the VCO in thepresence of a spurious signal is a jump of the centre frequency F1 ofthe VCO. This spurious signal appears when the various elements of themodulation circuit are activated, which is shown schematically in FIG.2A by the closing of a switch SWP. The centre frequency F1 is thenoffset towards a frequency F2 of pulsation ω2 and the frequency jump ΔFcan be written:ΔF=(ω2−ω1)/2π  (5)

When the VCO is combined with a PLL circuit, as shown in FIG. 1, thefrequency jump is compensated by the PLL circuit which brings the VCOback to its original centre frequency. The frequency jump then resultsin instantaneous frequency jumps ΔF (t).

This phenomenon has been observed by applying voltage pulses to themixer circuits IMIX and QMIX. The frequency F_(BB) of the baseband isthen zero and the harmonic H2 of the signal RFSX is equal to the naturalfrequency F1 of the VCO:K=2 and F _(BB)=0

H2=2F _(RF)+2*0

H2=2F _(RF) =F1   (6)

As shown in FIG. 3, it can then be seen that the voltage V1 delivered bythe VCO has frequency jumps upon each pulse sent on the channels IQ. Thefrequency jumps are due to the instantaneous activation of the spuriouspaths and the slow re-establishment of the original frequency by actionof the phase locked loop. The amplitude of the voltages applied to thechannels I and Q determines the phase and the amplitude of the outputsignal RFSX and therefore of the disturbing harmonic H2 reinjected intothe VCO.

The frequency jumps can be characterised mathematically with referenceto the VCO model described above and represented in FIG. 2A. Byconsidering that the expression of the spurious signal is Vsp*e^(jφ) andthat it has an amplitude Vsp and a phase φ, the loop equation is asfollows:V1=V1 Gi F (ω)+Vsp e ^(jφ)  (7)

By considering now that the spurious signal is the output voltage V1 ofthe VCO that is reinjected into the core of the VCO by a spurious pathhaving a transfer function α e^(jφ), when the switch SWP is closed, theloop equation can be written as:V1=V1 Gi F (ω)+V1 α e^(jφ)  (8)with:α=Vsp/V1   (9)

By expressing the spurious transfer function in Cartesian coordinates:b=Re (α e ^(jφ))   (10)d=Im (α e ^(jφ))   (11)the term ω2 can be found which meets the loop equation:ω2=½[d/(1-b)RC+√{square root over ( )} [d/(1-b)RC)²+4/LC]]  (12)

Therefore, it appears that the term “d” is zero and that the pulsationω2 is equal to ω1 if the phase of the signal reinjected is zero inrelation to the phase of the voltage V1 (φ=0). In this case, thefrequency jump ΔF is zero. If, on the contrary, φ=90° (maximum phase ofthe spurious signal) then b=0, d=α and:ω2=½[α/RC+√{square root over ( )}[(α/RC)²+4/LC]  (13)i.e.:ΔF=[½[α/RC+√{square root over ( )}[(α/RC)²+4/LC[−ω1[/2π  (14)

These relations between the phase of the disturbing signal have beenconfirmed by computer simulations conducted using the VCO model. Bysimulating the injection into the VCO of a spurious signal of variablephase, a curve like the one represented in FIG. 4 could be traced. Itcan be seen on this Figure that the centre frequency F1 of the VCO hasjumps varying between two maxima +ΔFmax and −ΔFmax depending on thephase φ of the spurious signal, and has a zero value when the phase ofthe spurious signal is zero.

In fact, the equations allowing the frequency jumps to be characterisedare confirmed by the experimental observations and by the computersimulations. It is therefore known as far as the frequency jumps areconcerned that there is an exact concordance between the electricalcharacteristics (amplitude and phase) of the spurious signal that entersthe core of the VCO and the disturbing phenomenon.

Second Disturbing Phenomenon: Spurious Lines in the Spectrum ofFrequencies of the VCO

This phenomenon is highlighted by applying for example to the channel Iand the channel Q two pure sine curves of frequency F_(BB) in phasequadrature. As represented in FIG. 5A, a single-sideband signal H1, offrequency F_(RF)+F_(BB) (component H1) then appears at the output of thecircuit TXCT. Harmonics H2, H3 . . . also appear. Traces of the carrierF_(RF) and traces of the image signal F_(RF)−F_(BB) that is neutralisedor at least attenuated by the quadrature phase modulation can also bedistinguished.

As K is here equal to 2, the harmonic H2 is the component of themodulated signal that is the most proximate to the centre frequency F1of the VCO. This harmonic of frequency 2F_(RF)+2F_(BB), i.e.,F1+2F_(BB), is reinjected into the VCO by spurious paths. By observingthe output of the VCO by means of a spectrum analyser, the appearance,in addition to the signal V1 of frequency F1, of a spurious line SH2 ofthe same frequency as the harmonic H2 can be seen, as shown in FIG. 5B.

The appearance, to the left of the centre frequency F1 of the VCO, of animage spurious line ISH2 of frequency 2F_(RF)−2F_(BB) (i.e., F1+2F_(BB))can also be seen.

The presence of this line ISH2 can be explained in a manner shown inFIGS. 6A and 6B. A VCO is a system that, by design, is limited inamplitude and operates like a clipping amplifier in relation to thespurious signal. However, the spurious signal injected, of frequencyF1+2F_(BB), is the vector sum of a vector ↑V1 of frequency F1 and aphasor ↑V2 of frequency 2F_(BB). The amplitude of the vector ↑V1 isdefined by the conditions of oscillation of the VCO and cannot beexceeded. Thus, the amplitude component of the phasor ↑V2 of frequency2F_(BB) is removed by the VCO. As represented in FIG. 6A, the mechanismfor removing the amplitude component transforms the phasor ↑V2 into aphasor ↑V2′ oriented according to an axis AA′ that is perpendicular toan axis BB′ according to which the vector ↑V1 is oriented. The vectorialbreakdown of this phasor ↑V2′ gives two vectors ↑V3, ↑V3′ of amplitudeV2/2 in opposite phase and of respective frequencies 2F_(BB) and−2F_(BB). Another phenomenon can be added that is due to the presence ofthe PLL circuit, which forces the spurious vector ↑V1+↑V3+↑V3′ to be inphase with the central vector ↑V1 of the VCO. Thus, as shown in FIG. 6B,the phase of the vector resulting from the sum of the two vectors ↑V3,↑V3′ is kept constant and equal to that of the central vector ↑V1. It istherefore aligned with the rotating axis BB′. This phenomenon occurs inthe bandwidth of the phase locked loop.

In summary, the existence of the image line ISH2 is due to the amplitudestresses and to the phase stresses that are exerted on the spuriousmodulation signal once the latter is injected into the VCO.

These disturbances are also represented on FIG. 5B, in which firstly acurve φnoise representing the phase noise generated by the thermalagitation in the resistor of the VCO has been traced, and secondly acurve Cm described below representing the variations in the amplitude ofeach spurious line when the frequency F_(BB) varies.

In real conditions of use, the signals I and Q in the baseband are notpure sine curves but complex digital signals the instantaneous frequencyof which varies permanently inside the baseband. Consequently, the twospurious lines represented in FIG. 5B move constantly and a wholespectrum of spurious signals can be observed.

The amplitude of the two spurious lines has been measured experimentallyfor various values of the frequency F_(BB) of the signals I and Q. Thecurve Cm represented in FIG. 5B is represented in greater detail in FIG.7, and is traced here for a single spurious line SH2, ISH2. Thehorizontal axis of the drawing represents the logarithm of an offsetfrequency Foff equal to the difference between the frequency 2F_(BB) ofthe spurious line and the centre frequency F1 of the VCO. The verticalaxis is the amplitude of the spurious lines in dBc. This experimentaldrawing shows that the amplitude of the spurious lines SH2, ISH2 has amaximum when the frequency 2F_(BB) falls on the resonance frequency Fcof the loop PLL (which corresponds to the cut-off frequency of the loopfilter LOOPF as modified by the loop gain). When the frequency 2F_(BB)increases above the frequency Fc, the amplitude of the spurious linesdecreases with a slope of 20 db/decade. The curve Cm also decreasesrapidly inside the bandwidth of the PLL circuit, since the spurioussignal is rejected by the loop gain.

It is important to note that these observations confirm that it issufficient to neutralise the disturbing effects of the harmonic the mostproximate to the centre frequency F1 of the VCO, the frequency of whichis in the vicinity of the limits of the bandwidth of the VCO, since thedisturbing effect of the higher-ranking harmonics is low due to theattenuation of 20 dB per decade.

It can also be noted that the amplitude of the harmonics decreasesrapidly going towards the high-ranking harmonics. Therefore, inpractice, the radio frequency modulation circuits that are the mostsensitive to the injection pulling are the circuits in which K=2, forexample the transmission circuits provided for the DCS network (DigitalCellular System). The transmission circuits for the GSM network (“GlobalSystem for Mobile Communication”) have a K ratio generally equal to 4and their voltage controlled oscillators are less sensitive to theinfluence of the harmonic of rank 4, which nonetheless remainsproblematic.

The phase noise observed in FIG. 5B can be characterised theoreticallywith reference to the diagram in FIG. 2B. According to an acceptedtheory, the natural phase noise of a VCO, in the absence of an externalspurious signal, is generated by the thermal agitation of the resistor Rof the VCO (reactive part F (ω)). The natural phase noise expressed indBc/Hz (noise/carrier signal ratio in decibels, i.e., here anoise/amplitude ratio of the centre frequency of the VCO) obeys thefollowing relation:Φout(ω)=20 log[(1/√{square root over ( )}2)*(√{square root over ()}(4kTR)V1 rms)/(1-F(ω)/R)] dBc/Hz   (15)“V1 rms” being the oscillation amplitude (in Volt rms) of the VCO in theabsence of a spurious signal, k being the Boltzmann constant, T thetemperature in Kelvin, and R the resistor of the reactive part F (ω)expressed in Ohms.

In other terms, the phase noise appears like the ratio between theamplitude of the thermal noise and the amplitude V1 rms of the centrefrequency of the VCO seen through the transfer function F (ω).

By considering that the source of noise is a spurious signal injectedinto the VCO, and by designating its effective amplitude by “Vsprms” (inVolt rms), a similar reasoning shows that the phase noise Φout due tothe injection of the spurious voltage (and which is therefore now rathera level of spurious line) obeys the following relation:Φout(ωoff)=20 log[ 1/2*(Vsprms/V1rms)/(1−(F(ωoff)/R)) dBc   (16)ωoff being the pulsation corresponding to the offset frequency Foff(Foff=F1−2F_(BB) or F1+2F_(BB) when K=2)

The relation 16 confirms the fact that the level of spurious lineaccording to the offset frequency has a linear decrement of 20 dB perdecade outside the bandwidth of the loop PLL, which has beenexperimentally observed above (FIG. 7).

By inverting the relation 16, the following is obtained:Vsp(Φout)=2 10^(Φout/20) |1−(F (ω)/R)|V1   (17)

By measuring the level of spurious line Φout in dBc at the output of theVCO, it is possible to find out the level of the spurious signal (ineffective voltage) entering the core of the VCO.

Conclusions About the Experimental and Theoretical Studies andFormulation of a Technical Problem

To summarise the above, mathematical equations confirmed by experimentalobservations and by computer simulations, show that the two disturbingphenomena that are the frequency jumps and the spurious modulation (orphase noise) are attributable to a single cause taking the form of aspurious signal, of determined amplitude and phase.

With reference to FIG. 8, a model of a technical problem and a model ofa solution to this technical problem can therefore be formulated asfollows: in the circuit represented in FIG. 8, the modulation stages IQof the modulation circuit TXCT modulate a carrier F_(RF) proportional tothe centre frequency F1 of the VCO, and more particularly equal to F1/2(K=2) or to F1/4 (K=4) depending on the applications. The modulatedsignal passes in a modulation circuit produced with real components andtherefore inevitably imperfect, and thus has a slight non-linearityimperfection. The output signal delivered by a non-linear circuit can bemodelled by a polynomial:F(t)=b0+b1x(t)+b2x(t)² +b3x(t)³ +b4x(t)⁴   (18)i.e.:F(t)=b0+H1+H2+H3+H4+ . . .   (19)b0 being the DC offset of the output signal, H1 being the fundamental orwanted part of the output signal and b1 the gain on the wanted signal,H2 being the second harmonic and b2 the amplitude of the secondharmonic, etc.

Harmonics are thus generated and at least one harmonic falls within thebandwidth of the VCO and disturbs its operation, which corresponds to aninjection of a spurious signal. The spurious harmonic that disturbs theVCO the most is the one that is the most proximate to the oscillationfrequency of the VCO, i.e., the harmonic H2 when K=2 or the harmonic H4when K=4 . . .

The spurious harmonic is propagated to the core of the VCO by manyspurious paths (magnetic induction, electromagnetic radiations, pathspassing through the substrate, paths passing through the power supplylines . . . ) each having their own transfer function, represented inFIG. 8 by blocks SA1, SA2, SA3 . . . SAn.

Whatever the number of spurious paths, the spurious signals A1, A2, A3 .. . An are added up and there is therefore one resulting spurious signalAnet that is the result of the vector sum of the vectors A1, A2 . . . Anand which has a determined amplitude and phase:Anet=A0 ejφ  (20)General Features of the Method According to the Present Invention

According to the present invention, provision is thus made to injectinto the VCO a spurious signal forming a compensation signal Bcomp,having the same amplitude as the signal Anet but in opposite phase withthe signal Anet (i.e., a phase angle of 180°), such that Anet+Bcomp=0.

The signal Bcomp is delivered by a compensation circuit COMPCT accordingto the present invention, to which a determined signal is applied atinput. The circuit COMPCT adjusts the phase and the amplitude of thedetermined signal that is supplied to it at input, to obtain thecompensation signal Bcomp. Various examples of embodiments of thiscircuit will be described below.

The determined signal to be supplied to the circuit COMPCT mustcorrespond in frequency to the harmonic H2 or H4 the disturbing effectsof which are to be neutralised. As it will be understood from theexamples below, it is advantageous for this signal to be the disturbingharmonic itself, which is easy to extract from the output stages of themodulation circuits, such as certain points of the output amplifierRFAMP for example that are rich in harmonics.

It should be noted that, in certain applications, it can happen that anode rich in harmonics H2 or H4 is not available or is not accessible.In this case, a harmonic generating circuit will be produced, by takingoff in the circuit TXCT the carrier signal RFSX after the modulationstages (i.e., the modulated signal F_(RF)) and by applying this signalto non-linear components.

Finally, the point of injection of the compensation signal into the VCOmust also be determined. Various options can be provided and referencewill be made as an example to FIG. 9, which is the partial wiringdiagram of a classical VCO. This VCO is here of the balanced type andhas a left part VCOL (“VCO left”) and a right part VCOR (“VCO righ”)that operate in opposite phase for the generation of the output signalV1. Various points P1L, P1R, P2L, P2R, P3L, P3R of injection of thecompensation signal Bcomp are represented by circles in dotted lines.

The signal Bcomp can be injected onto active component controlterminals, such as onto bases of bipolar transistors T1, T2 (points P1Lor P1R) for example through a capacitor aiming to avoid the introductionof a spurious DC signal. The signal Bcomp can also be injected ontoterminals of passive components, such as cathodes of capacitors C1, C2(points P2L, P2R) for example, the anodes of which receive a biasvoltage Vbias. The injection of the signal Bcomp can also be carried outby inductive coupling, by means of an injection inductor Lc coupled withan inductor L1 of the VCO for example. The signal Bcomp is then appliedto one of the ends of the inductor Lc (points P3L, P3R) the other endbeing grounded.

Now various examples of embodiments of a compensation circuit accordingto the present invention will be described. In the followingdescription, it will be assumed, as above, that the compensation signalaims to neutralise the disturbing effects of the second harmonic H2 ofthe modulated signal delivered by the circuit TXCT.

Examples of Compensation Circuits

FIG. 10 represents a first embodiment of a compensation circuit COMPCT1according to the present invention. The circuit COMPCT1 comprises aphase shift network PSN receiving the harmonic H2 at input. The outputof the circuit PSN is applied to an amplitude attenuator ATTC. Theoutput of the attenuator ATTC delivers the signal Bcomp and is appliedto the part VCOL or to the part VCOR of the VCO, at one point ofinjection to be chosen for example out of the points of injectionP1L/P1R, P2L/P2R, P3L/P3R described above.

The harmonic H2 is taken off at one node of the output amplifier RFAMPrich in harmonics and that does not have the fundamental H1 (wantedsignal), such as at one emitter node of two bipolar transistors forexample, and through a capacitor aiming to remove any DC offsets of thesignal present on this node.

As represented in FIG. 11, the circuit PSN comprises one or two cells RCin series, here two cells CELL1, CELL2. Each cell CELL1, CELL2 comprisesa first group RC formed by a capacitor and a resistor that areadjustable in parallel in series with a second group RC also formed by acapacitor and a resistor adjustable in parallel. The output point ofeach cell is the midpoint of the two groups RC. Depending on the valuegiven to these elements, the circuit PSN enables the desired phase leador lag to be applied to the harmonic H2. Therefore, the harmonic H2taken off with a determined phase φ in the amplifier RFAMP is deliveredby the circuit PSN with a corrected phase φ′.

As represented in FIG. 12, the attenuator circuit ATTC is for example anadjustable resistive dividing bridge, that corrects the amplitude of theharmonic H2 (φ′) to deliver the signal Bcomp (φ′).

The circuits PSN and ATTC are adjusted during an electric test stepprior to commissioning the circuit RFCT. The phase and amplitude valuesare adjusted empirically by applying test signals to the circuit RFCT,until the output of the VCO delivers a “clean” signal without thespurious phenomena described above, obviously as far as is possible andwithin the limits of the accepted tolerances, since a totalneutralisation of the disturbing effects is in practice veryunrealistic.

This embodiment of the compensation circuit according to the presentinvention is preferentially intended to be implemented in the form of adiscrete component circuit. Now, with reference to FIGS. 13 and 17, twoother embodiments COMPCT2, COMPCT3 of the compensation circuit accordingto the present invention will be described that are provided to bepreferentially implemented in an RF integrated circuit.

The compensation circuits represented in these Figures are digitallyadjustable and the adjustment values, once determined, are recorded in aregister NVREG. The outputs of the register NVREG are applied to adigital to analog converter DAC with several ways, that delivers aplurality of analog signals to electrically adjustable capacitors of theVARICAP type.

The circuit COMPCT2 represented in FIG. 13 is of the single ended typeand receives the harmonic H2 at input, which is taken off in the mannerdescribed above. The circuit COMPCT2 comprises a quadrature generatorQGEN1 and four phase-shifters/attenuators IAT1, IBAT1, QAT1, QBAT1driven by the outputs of the converter DAC. These are pure attenuatorsthat control four sine-wave signals, respectively I, IB (or /I, i.e., Ishifted by 180°), Q (sine curve in quadrature with I) and QB (or /Q,i.e., Q shifted by 180°). The sum of the four sine curves gives a newsine curve having an amplitude and a phase that is the result of thevector sum of the four signals.

The generator QGEN1 respectively delivers, to two distinct outputs, theharmonic H2 phase-shifted by +45° and the harmonic H2 phase-shifted by−45°. The harmonic H2 phase-shifted by +45° is applied to theattenuators IAT1 and IBAT1 while the harmonic H2 phase-shifted by −45°is applied to the attenuators QAT1 and QBAT1. The outputs of theattenuators IAT1 and QAT1 are added up to form a signal Bcomp1 that isapplied to the part VCOL of the VCO, at one of the points of injectionP1L, P2L or P3L described above. The outputs of the attenuators IBAT1and QBAT1 are added up to form a signal Bcomp2 that is applied to thepart VCOR of the VCO, at one of the points of injection P1R, P2R, P3Rdescribed above.

As shown in FIG. 14, the quadrature generator QGEN1 comprises forexample a cell RC that phase shifts the harmonic H2 by +45° and acellule CR in parallel with the cell RC, that phase shifts the harmonicH2 by −45°.

As shown in FIG. 15, each attenuator IAT1, IBAT1, QAT1, QBAT1 comprisesa capacitive dividing bridge formed by two VARICAP capacitors eachdriven by an output of the converter DAC, said output delivering a DCsignal required to control these elements. The bias system of theVARICAP capacitors is well known by those skilled in the art and willnot be described for the sake of simplicity.

FIG. 16 is a phase diagram with four dials showing the phase andamplitude correction range provided by the attenuators according to thepresent invention. When the attenuators IAT1 and QAT1 are active and theattenuators IBAT1, QBAT1 are deactivated (high impedance state), thephase of the signal Bcomp is adjustable within the limits of the firstdial, i.e., between 0 and 90°, and the amplitude of the signal Bcomp isdetermined by the ratio between the values of the capacitors that formeach attenuator. When the attenuators IBAT1 and QAT1 are active and theattenuators IAT1, QBAT1 are deactivated, the phase of the signal Bcompis adjustable within the limits of the second dial, i.e., between 90°and 180°. When the attenuators IBAT1 and QBAT1 are active and theattenuators IAT1, QAT1 are deactivated, the phase of the signal Bcomp isadjustable within the limits of the third dial, i.e., between 180° and270°. When the attenuators IAT1 and QBAT1 are active and the attenuatorsIAT1, QAT1 are deactivated, the phase of the signal Bcomp is adjustablewithin the limits of the fourth dial, i.e., between 270° and 0°.

The circuit COMPCT3 represented in FIG. 17 is of the balanced type andreceives at input, in addition to the harmonic H2, a harmonic /H2phase-shifted by 180°. The harmonic H2 is taken off as described aboveon an emitter node of two transistors of the amplifier RFAMP. Theharmonic /H2 is taken off on a collector node of the same transistors.

The circuit COMPCT3 comprises a balanced-type quadrature generator QGEN2and four phase-shifters attenuators IAT2, IBAT2, QAT2, QBAT2 driven bythe converter DAC, each comprising a first and a second output. Thegenerator QGEN2 receives the harmonics H2, /H2 and respectivelydelivers, to four distinct outputs, the harmonic H2 phase-shifted by 0°,by +90°, by +180° and by +270°. The harmonics H2 phase-shifted by 0° andby 180° are both applied to the attenuators IAT2 and IBAT2. Theharmonics H2 phase-shifted by 90° and 270° are both applied to theattenuators QAT2 and QBAT2.

The first outputs of the attenuators IAT2, IBAT2, QAT2, QBAT2 are addedup to form a signal Bcomp1′ that is applied to the part VCOL of the VCO,at one point of injection P1L, P2L or P3L. The second outputs of theattenuators IAT2, IBAT2, QAT2, QBAT2 are added up to form a signalBcomp2′ in opposite phase with Bcomp1′, which is applied to the partVCOL of the VCO, at one point of injection P1R, P2R or P3R.

As shown in FIG. 18, the generator QGEN2 comprises a balanced bridge ofcapacitors and resistors, in which the variations in temperature andprocess (variations in the characteristics of the elements with themanufacturing process) are reduced. Therefore this generator ensures arelative phase shift of 90° between each of its outputs, whatever thevariation of the resistors and capacitors with the temperature or withthe manufacturing process, and the working frequency F1.

The generator QGEN2 can also be produced in the form of a POLYPHASEfilter, so as to further reduce the effects of variations intemperature, process and working frequency.

As shown in FIG. 19, each attenuator IAT1, IBAT1, QAT1, QBAT1 comprisesa balanced capacitive dividing bridge with two inputs and two outputs,formed by three adjustable VARICAP capacitors each driven by one outputof the converter DAC delivering a bias voltage. As above, the control ofthe bias voltages of the adjustable VARICAP capacitors will not bedescribed for the sake of simplicity. The first adjustable capacitor isarranged between the first input and the first output. The secondadjustable capacitor is arranged between the second input and the secondoutput. The third adjustable capacitor is arranged between the twooutputs.

The operation in phase and amplitude of the compensation circuit COMPCT3is similar in principle to the circuit COMPCT2 described above, andoffers the added advantage of being less sensitive to the variations intemperature and process and of being more accurate.

It will be understood by those skilled in the art that various otheralternatives and embodiments of the present invention may be made likethe generation of active quadrature, the use of POLYPHASE filter . . .Moreover, electrically adjustable resistors may be used instead of theVARICAP capacitors.

Although the description above focuses mainly on describing acompensation circuit of the disturbing effects of a harmonic of secondor of fourth rank, the scope of application of the present invention isobviously not limited to these examples, as K may be equal to 1(frequency of the VCO equal to the frequency RF), to 4, etc.Furthermore, although it was considered at the end of experimentalobservations and theoretical calculations that it is sufficient inpractice to neutralise the effects of a noise having a single origin,which is the result of the vector sum of the signals delivered via allthe spurious paths, it goes without saying that certain applications orcertain circuit architectures may require compensating the disturbingeffects of spurious signals of different origins (sent, for example,before or after a variable gain amplifier). In this case, twocompensation signals must be provided, and it is preferable for eachnoise of different origin to be treated by a dedicated compensationcircuit. In this case, two or more independently adjustable compensationsignals, which can be added up upstream from their point of injection,are injected into the VCO.

Finally, although the present invention has been described above inrelation with a phase modulation IQ circuit, it goes without saying thatthe scope of application of the present invention also relates to theother modulation circuits, particularly amplitude modulation AM circuitsand phase and amplitude modulation circuits, which also generatespurious harmonics.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. An RF circuit comprising: a voltage controlled oscillator deliveringan RF signal; a phase locked loop to control the voltage controlledoscillator; a modulation circuit receiving the RF signal and deliveringa modulated signal comprising at least one harmonic component of afrequency equal or proximate to that of the RF signal delivered by thevoltage controlled oscillator, the harmonic component being capable ofdisturbing the operation of the voltage controlled oscillator byinjection pulling; an injection pulling compensation circuit comprisingone input receiving at least the disturbing harmonic component and meansfor modifying the phase and the amplitude of the harmonic component todeliver an injection pulling compensation signal; and means forinjecting the compensation signal into the voltage controlledoscillator.
 2. The RF circuit according to claim 1, wherein thecompensation circuit is amplitude and phase adjusted such that thecompensation signal injected into the voltage controlled oscillator hasan amplitude substantially equal to the amplitude of a spurious signalresulting from the involuntary injection into the voltage controlledoscillator, by at least one spurious path, of the disturbing harmoniccomponent, and a phase opposite that of the spurious signal.
 3. The RFcircuit according to claim 1 wherein the compensation circuit a singleended circuit that delivers a compensation signal having a uniquecomponent that is injected at one point of the voltage controlledoscillator.
 4. The RF circuit according to claim 1 wherein thecompensation circuit a single ended circuit that delivers a compensationsignal having two components that are injected at two different pointsof the voltage controlled oscillator.
 5. The RF circuit according toclaim 1 wherein the compensation circuit a balanced circuit thatdelivers a compensation signal having two components in opposite phasethat are injected at two different points of the voltage controlledoscillator.
 6. The RF circuit according to claim 1 wherein thecompensation circuit receives at input a harmonic component taken off inthe modulation circuit.
 7. The RF circuit according to claim 6 whereinthe compensation circuit receives at input a harmonic component takenoff in an output amplifier of the modulation circuit.
 8. The RF circuitaccording to claim 1 wherein the compensation circuit receives at inputa harmonic component delivered by a harmonic generating circuit distinctfrom the modulation circuit.
 9. The RF circuit according to claim 1wherein the compensation circuit comprises a phase-shift circuit tomodify the phase of the harmonic component received at input.
 10. The RFcircuit according to claim 1 wherein the compensation circuit comprisesa phase-shift circuit receiving the disturbing harmonic component anddelivering two phase quadrature signals.
 11. The RF circuit according toclaim 1 wherein the compensation circuit comprises a phase-shift circuitreceiving the disturbing harmonic component and delivering phasequadrature and opposite phase signals.
 12. The RF circuit according toclaim 11 wherein the phase-shift circuit comprises a balanced bridge ofresistors and capacitors that is quite insensitive to the temperature.13. The RF circuit according to claim 1 wherein the compensation circuitcomprises at least one attenuator circuit to modify the amplitude of theharmonic component received at input.
 14. The RF circuit according toclaim 13 wherein the attenuator circuit comprises adjustable resistorsor capacitors or a combination of these elements.
 15. The RF circuitaccording to claim 13, comprising a group of at least two attenuatorcircuits the outputs of which are added up to control the phase and theamplitude of the compensation signal.
 16. The RF circuit according toclaim 15, comprising a group of attenuator circuits having their outputsadded up and receiving at input phase quadrature signals coming from thedisturbing harmonic component.
 17. The RF circuit according to claim 15,comprising a group of attenuator circuits having their outputs added upand receiving at input phase quadrature and opposite phase signalscoming from the disturbing harmonic component.
 18. The RF circuitaccording to claim 15 wherein an attenuator circuit compriseselectrically adjustable capacitors or electrically adjustable resistors,which are adjusted by analog signals delivered by a digital to analogconverter.
 19. The RF circuit according to claim 18 wherein digital datafor adjusting the capacitors of the attenuator circuit are stored inmemory cells and are applied to the digital to analog converter.
 20. TheRF circuit according to claim 1 wherein the compensation signal isinjected onto one terminal of an active component of the voltagecontrolled oscillator.
 21. The RF circuit according to claim 1 whereinthe compensation signal is injected onto one terminal of a passivecomponent of the voltage controlled oscillator.
 22. The RF circuitaccording to claim 1 wherein the means for injecting the compensationsignal comprise an injection inductor coupled to an inductor of thevoltage controlled oscillator.
 23. A method for stabilising theoperation of a voltage controlled oscillator that sends an RF signal andis driven by a phase locked loop comprising: receiving through at leastone spurious path a harmonic component of a frequency equal or proximateto that of the RF signal sent, capable of disturbing the operation ofthe voltage controlled oscillator by injection pulling; andinjectioning, into the voltage controlled oscillator, an injectionpulling compensation signal, the phase and the amplitude of which areadjusted so as to neutralise the disturbing effects of the harmoniccomponent.
 24. The method according to claim 23 wherein the compensationsignal is amplitude and phase adjusted so as to have an amplitudesubstantially equal to the amplitude of a spurious signal resulting fromthe involuntary injection into the voltage controlled oscillator, by atleast one spurious path of the disturbing harmonic component, and aphase opposite that of the spurious signal.
 25. The method according toclaim 23, comprising the single ended injection, at one point of thevoltage controlled oscillator, of a compensation signal having a uniquecomponent.
 26. The method according to claim 23 comprising the injectionof a compensation signal having two components, and the single endedinjection of these components at two different points of the voltagecontrolled oscillator.
 27. The method according to claim 23, comprisingthe injection of a compensation signal having two components in oppositephase, and the injection of these two components at two different pointsof the voltage controlled oscillator.
 28. The method according to claim23, wherein the compensation signal is generated from at least oneharmonic component taken off in the modulation circuit.
 29. The methodaccording to claim 28 wherein the compensation signal is generated fromat least one harmonic component taken off in an amplifier of amodulation circuit from which the disturbing harmonic component is sent.30. The method according to claim 23 wherein the compensation signal isgenerated from one harmonic component produced by a harmonic generatingcircuit.
 31. The method according to claim 23 wherein the phase of thecompensation signal is adjusted by means of a phase-shift circuit. 32.The method according to claim 31 wherein the amplitude of thecompensation signal is adjusted by means of an attenuator circuitcomprising adjustable resistors or capacitors or a combination of theseelements.
 33. The method according to claim 23 wherein the amplitude andthe phase of the compensation signal are adjusted by means of a group ofat least two attenuator circuits the outputs of which are added up. 34.The method according to claim 33 wherein the amplitude and the phase ofthe compensation signal are adjusted by means of a group of attenuatorcircuits having their outputs added up and receiving at input phasequadrature signals coming from the disturbing harmonic component. 35.The method according to claim 33 wherein the amplitude and the phase ofthe compensation signal are adjusted by means of a group of attenuatorcircuits having their outputs added up and receiving at input phasequadrature and opposite phase signals coming from the disturbingharmonic component.
 36. The method according to claim 35 wherein thephase quadrature and opposite phase signals are generated by means of aphase-shift circuit comprising a balanced bridge of resistors andcapacitors that is quite insensitive to the temperature.
 37. The methodaccording to claim 33 wherein an attenuator circuit compriseselectrically adjustable capacitors or electrically adjustable resistorsthat are adjusted by analog signals coming from adjustment digital data.38. The method according to claim 37 wherein the adjustment digital dataare stored in memory cells.
 39. The method according to claim 23 whereinthe compensation signal is injected onto one terminal of an activecomponent of the voltage controlled oscillator.
 40. The method accordingto claim 23 wherein the compensation signal is injected onto oneterminal of a passive component of the voltage controlled oscillator.41. The method according to claim 23 wherein the compensation signal isinjected by inductive coupling.